Method and system for production of liquid natural gas

ABSTRACT

A process and system for liquefying a hydrocarbon gas is provided. The hydrocarbon feed gas is pre-treated to remove sour species and water therefrom. The pre-treated feed gas is then passed to a refrigeration zone where it is cooled and expanded to produce a hydrocarbon liquid. A closed loop single mixed refrigerant provides most of the refrigeration to the refrigeration zone together with an auxiliary refrigeration system. The auxiliary refrigeration system and closed loop single mixed refrigerant are coupled in such a manner that waste heat generated by a gas turbine drive of the compressor in the closed loop single mixed refrigerant drives the auxiliary refrigeration system and the auxiliary refrigeration system cools the inlet air of the gas turbine. In this way, substantial improvements are made in the production capacity of the system.

FIELD

The present invention relates to a method and system for production ofliquid natural gas. In particular, the present invention relates to aprocess and system for liquefying a hydrocarbon gas, such as natural gasor coal seam gas.

BACKGROUND

The construction and operation of a plant for treating and liquefying ahydrocarbon gas, such as natural gas or coal seam gas, and produceliquefied methane or LNG involves vast capital and operationalexpenditure. In particular, with increased sensitivity to environmentalissues and regulations pertaining to green house gas emissions, thedesign of such a plant must seek to incorporate features which increasefuel efficiency and reduce emissions where possible.

SUMMARY

In its broadest aspect, the invention provides a process and system forliquefying a hydrocarbon gas, such as natural gas or coal seam gas.

Accordingly, in a first aspect, the present invention provides a processfor liquefying a hydrocarbon gas comprising the steps of:

-   -   a) pre-treating a hydrocarbon feed gas to remove sour species        and water therefrom;    -   b) providing a refrigeration zone, wherein refrigeration in the        refrigeration zone is provided by circulating a mixed        refrigerant from mixed refrigerant system and an auxiliary        refrigerant from an auxiliary refrigeration system through the        refrigeration zone;    -   c) coupling the mixed refrigerant system and the auxiliary        refrigeration system in a manner whereby the auxiliary        refrigeration system is driven, at least in part, by waste heat        generated by the mixed refrigerant; and    -   d) passing the pre-treated feed gas through the refrigeration        zone where the pre-treated feed gas is cooled and expanding the        cooled feed gas to produce a hydrocarbon liquid.

In one embodiment of the invention, the step of circulating a mixedrefrigerant through the refrigeration zone comprises:

-   -   a) compressing the mixed refrigerant in a compressor;    -   b) passing the compressed mixed refrigerant through a first heat        exchange pathway extending through the refrigeration zone where        the compressed mixed refrigerant is cooled and expanded to        produce a mixed refrigerant coolant;    -   c) passing the mixed refrigerant coolant through a second heat        exchange pathway extending through the refrigeration zone to        produce a mixed refrigerant; and    -   d) recirculating the mixed refrigerant to the compressor.

In another embodiment of the invention, the step of passing thepre-treated feed gas through the refrigeration zone comprises passingthe pre-treated feed gas through a third heat exchange pathway in therefrigeration zone.

In still another embodiment of the invention, the step of circulatingthe auxiliary refrigerant through the refrigeration zone comprisespassing the auxiliary refrigerant through a fourth heat exchange pathwayextending through a portion of the refrigeration zone. The second andfourth heat exchange pathways extend in counter current heat exchangerelation to the first and third heat exchange pathways.

Advantageously, the inventors have discovered that heat produced in thecompressing step by a gas turbine drive of the compressor, which wouldotherwise be considered as waste heat, can be utilised in the process toproduce steam in a steam generator. The steam may be used to power asingle steam turbine generator and produce electrical power which drivesthe auxiliary refrigeration system.

Accordingly, in a preferred embodiment of the invention, the processfurther comprises driving the auxiliary refrigeration system at least inpart by waste heat produced from the compressing step of the process ofthe present invention.

In another preferred embodiment of the invention, the process furthercomprises cooling inlet air of a gas turbine directly coupled to thecompressor with the auxiliary refrigerant. Preferably, the inlet air iscooled to about 5° C.-10° C. The inventors have estimated that coolingthe inlet air of the gas turbine increases the compressor output by15%-25%, thus improving the production capacity of the process sincecompressor output is proportional to LNG output.

In one embodiment of the invention, the step of compressing the mixedrefrigerant increases the pressure thereof from about 30 to 50 bar.

When the mixed refrigerant is compressed its temperature rises. In afurther embodiment, the process comprises cooling the compressed mixedrefrigerant prior to passing the compressed mixed refrigerant to thefirst heat exchange pathway. In this way the cooling load on therefrigeration zone is reduced. In one embodiment, the compressed mixedrefrigerant is cooled to a temperature less than 50° C. In the preferredembodiment, the compressed mixed refrigerant is cooled to about 10° C.

In another embodiment, the step of cooling the compressed mixedrefrigerant comprises passing the compressed mixed refrigerant from thecompressor to a heat exchanger, in particular an air- or water-cooler.In an alternative embodiment of the invention the cooling step comprisespassing the compressed mixed refrigerant from the compressor to the heatexchanger as described above, and further passing the compressed mixedrefrigerant cooled in the heat exchanger to a chiller. Preferably, thechiller is driven at least in part by waste heat, in particular wasteheat produced from the compressing step.

In one embodiment of the invention, the temperature of the mixedrefrigerant coolant is at or below the temperature at which thepre-treated feed gas condenses. Preferably the temperature of the mixedrefrigerant coolant is less than -150° C.

In one embodiment of the invention, the mixed refrigerant containscompounds selected from a group consisting of nitrogen and hydrocarbonscontaining from 1 to 5 carbon atoms. Preferably, the mixed refrigerantcomprises nitrogen, methane, ethane or ethylene, isobutane and/orn-butane. In one preferred embodiment the composition for the mixedrefrigerant is as follows in the following mole fraction percent ranges:nitrogen: about 5 to about 15; methane: about 25 to about 35; C2: about33 to about 42; C3: 0 to about 10; C4: 0 to about 20 about; and C5: 0 toabout 20. The composition of the mixed refrigerant may be selected suchthat composite cooling and heating curves of the mixed refrigerant arematched within about 2° C. of one another, and that the compositecooling and heating curves are substantially continuous.

In one embodiment of the invention, the hydrocarbon gas is natural gasor coal seam methane. Preferably, the hydrocarbon gas is recovered fromthe refrigeration zone at a temperature at or below the liquefactiontemperature of methane.

In a second aspect the invention provides a hydrocarbon gas liquefactionsystem comprising:

-   -   a) a mixed refrigerant;    -   b) a compressor for compressing the mixed refrigerant;    -   c) a refrigeration heat exchanger for cooling a pre-treated feed        gas to produce a hydrocarbon liquid, the refrigeration heat        exchanger having a first heat exchange pathway in fluid        communication with the compressor, a second heat exchange        pathway, and a third heat exchange pathway, the first, second        and third heat exchange pathways extending through the        refrigeration zone, and a fourth heat exchange pathway extending        through a portion of the refrigeration zone, the second and        fourth heat exchange pathways being positioned in counter        current heat exchange in relation to the first and third heat        exchange pathways;    -   an expander in fluid communication with an outlet from the first        heat exchange pathway and an inlet to the second heat exchange        pathway;    -   d) a recirculation mixed refrigerant line in fluid communication        with an outlet from the second heat exchange pathway and an        inlet to the compressor;    -   e) an auxiliary refrigeration system having an auxiliary        refrigerant in fluid communication with the fourth heat exchange        pathway;    -   f) a source of pre-treated feed gas in fluid communications with        an inlet of the third heat exchange pathway; and    -   g) a hydrocarbon liquid line in fluid communication with an        outlet of the third heat exchange pathway.

In one embodiment of the invention, the compressor is a single stagecompressor. Preferably, the compressor is a single stage centrifugalcompressor driven directly (without gearbox) by a gas turbine. In analternative embodiment, the compressor is a two stage compressor withintercooler and interstage scrubber, optionally provided with gearbox.

In another embodiment, the gas turbine is coupled with a steam generatorin a configuration whereby, in use, waste heat from the gas turbinefacilitates production of steam in the steam generator. In a furtherembodiment, the system comprises a single steam turbine generatorconfigured to produce electrical power. Preferably, the amount ofelectrical power generated by the single steam turbine generator issufficient to drive the auxiliary refrigeration system.

In another embodiment of the invention, the auxiliary refrigerantcomprises low temperature ammonia and the auxiliary refrigeration systemcomprises one or more ammonia refrigeration packages. Preferably the oneor more ammonia refrigeration packages are cooled by air coolers orwater coolers.

In a preferred embodiment, the auxiliary refrigeration system is in heatexchange communication with the gas turbine, the heat exchangecommunication being configured in a manner to effect cooling of inletair of the gas turbine by the auxiliary refrigeration system.

In a further embodiment of the invention, the system comprises a coolerto cool the compressed mixed refrigerant prior to the compressed mixedrefrigerant being received in the refrigeration heat exchanger.Preferably the cooler is an air-cooled heat exchanger, or a water-cooledheat exchanger. In an alternative embodiment of the invention, thecooler further comprises a chiller in sequential combination with theair- or water-cooled heat exchanger. Preferably, the chiller is drivenat least in part by waste heat produced from the compressor, inparticular by waste heat produced from the gas turbine drive.

In a still further embodiment of the invention, the hydrocarbon liquidin the hydrocarbon liquid line is expanded through an expander tofurther cool the hydrocarbon liquid.

Description of the Drawings

Preferred embodiments, incorporating all aspects of the invention, willnow be described by way of example only with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic flow chart of a process for liquefying a fluidmaterial, such as for example natural gas or CSG, in accordance with oneembodiment of the present invention; and

FIG. 2 is a composite cooling and heating curve for a single mixedrefrigerant and the fluid material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a process for cooling a fluidmaterial to cryogenic temperatures for the purposes of liquefactionthereof. Illustrative examples of a fluid material include, but are notlimited to, natural gas and coal seam gas (CSG). While this specificembodiment of the invention is described in relation to the productionof liquefied natural gas (LNG) from natural gas or CSG, it is envisagedthat the process may be applied to other fluid materials which may beliquefied at cryogenic temperatures.

The production of LNG is broadly achieved by pre-treating a natural gasor CSG feed gas to remove water, carbon dioxide, and optionally otherspecies which may solidify downstream at temperatures approachingliquefaction, and then cooling the pre-treated feed gas to cryogenictemperatures at which LNG is produced.

Referring to FIG. 1, the feed gas 60 enters the process at a controlledpressure of about 900 psi. Carbon dioxide is removed therefrom bypassing it through a conventional packaged CO₂ stripping plant 62 whereCO₂ is removed to about 50-150 ppm. Illustrative examples of a CO₂stripping plant 62 include an amine package having an amine contactor(eg. MDEA) and an amine re-boiler. Typically, the gas exiting the aminecontactor is saturated with water (eg. ˜70 lb/MMscf). In order to removethe bulk of the water, the gas is cooled to near its hydrate point (eg.˜15° C.) with a chiller 66. Preferably, the chiller 66 utilises coolingcapacity from an auxiliary refrigeration system 20. Condensed water isremoved from the cooled gas stream and returns to the amine package formake-up.

Water must be removed from the cooled gas stream to ≦1 ppm prior toliquefaction to avoid freezing when the temperature of the gas stream isreduced to below hydrate freezing point. Accordingly, the cooled gasstream with reduced water content (e.g. ˜20 lb/MMscf) is passed to adehydration plant 64. The dehydration plant 64 comprises three molecularsieve vessels. Typically, two molecular sieve vessels will operate inadsorption mode while the third vessel is regenerated or in standbymode. A side stream of dry gas exiting the duty vessel is used forregeneration gas. Wet regeneration gas is cooled using air and condensedwater is separated. The saturated gas stream is heated and used as fuelgas. Boil-off gas (BOG) is preferentially used as regeneration/fuel gas(as will be described later) and any shortfall is supplied from the drygas stream. No recycle compressor is required for regeneration gas.

The feed gas 60 may optionally undergo further treatment to remove othersour species or the like, such as sulphur compounds, although it will beappreciated that many sulphur compounds may be removed concurrently withcarbon dioxide in the CO₂ stripping plant 62.

As a result of pre-treatment, the feed gas 60 becomes heated totemperatures up to 50° C. In one embodiment of the present invention,the pre-treated feed gas may optionally be cooled with a chiller (notshown) to a temperature of about 10° C. to -50° C. Suitable examples ofthe chiller which may be employed in the process of the presentinvention include, but are not limited to, an ammonia absorptionchiller, a lithium bromide absorption chiller, and the like, or theauxiliary refrigeration system 20.

Advantageously, depending on the composition of the feed gas, thechiller may condense heavy hydrocarbons in the pre-treated stream. Thesecondensed components can either form an additional product stream, ormay be used as a fuel gas or as a regeneration gas in various parts ofthe system.

Cooling the pre-treated gas stream has the primary advantage ofsignificantly reducing the cooling load required for liquefaction, insome instances by as much as 30% when compared with the prior art.

The cooled pre-treated gas stream is supplied to a refrigeration zone 28through line 32 where said stream is liquefied.

The refrigeration zone 28 comprises a refrigerated heat exchangerwherein refrigeration thereof is provided by a mixed refrigerant and anauxiliary refrigeration system 20. Preferably, the heat exchangercomprises brazed aluminium plate fin exchanger cores enclosed in apurged steel box.

The refrigerated heat exchanger has a first heat exchange pathway 40 influid communication with the compressor 12, a second heat exchangepathway 42, and a third heat exchange pathway 44. Each of the first,second and third heat exchange pathways 40, 42, 44 extend through therefrigerated heat exchanger as shown in FIG. 1. The refrigerated heatexchanger is also provided with a fourth heat exchange pathway 46 whichextends through a portion of the refrigerated heat exchanger, inparticular a cold portion thereof. The second and fourth heat exchange42, pathways are positioned in counter current heat exchange in relationto the first and third heat exchange pathways 40, 44.

Refrigeration is provided to the refrigeration zone 28 by circulatingthe mixed refrigerant therethrough. The mixed refrigerant from arefrigerant suction drum 10 is passed to the compressor 12. Thecompressor 12 is preferably two parallel single stage centrifugalcompressors, each directly driven by gas turbines 100, in particular anaero-derivative gas turbine. Alternatively, the compressor 12 may be atwo stage compressor with intercooler and interstage scrubber. Typicallythe compressor 12 is of a type which operates at an efficiency of about75% to about 85%.

Waste heat from the gas turbines 100 may be used to generate steam whichin turn is used to drive an electric generator (not shown). In this way,sufficient power may be generated to supply electricity to all theelectrical components in the liquefaction plant, in particular theauxiliary refrigeration system 20.

Steam that is generated by waste heat from the gas turbines 100 may alsobe used to heat the amine re-boiler of the CO₂ stripping plant 62, forregeneration of the molecular sieves of the dehydration plant 64,regeneration gas and fuel gas.

The mixed refrigerant is compressed to a pressure ranging from about 30bar to 50 bar and typically to a pressure of about 35 to about 40 bar.The temperature of the compressed mixed refrigerant rises as aconsequence of compression in compressor 12 to a temperature rangingfrom about 120° C. to about 160° C. and typically to about 140° C.

The compressed mixed refrigerant is then passed through line 14 to acooler 16 to reduce the temperature of the compressed mixed refrigerantto below 45° C. In one embodiment, the cooler 16 is an air-cooled fintube heat exchanger, where the compressed mixed refrigerant is cooled bypassing the compressed mixed refrigerant in counter current relationshipwith a fluid such as air, or the like. In an alternative embodiment, thecooler 16 is a shell and tube heat exchanger where the compressed mixedrefrigerant is cooled by passing the compressed mixed refrigerant incounter current relationship with a fluid, such as water, or the like.

The cooled compressed mixed refrigerant is passed to the first heatexchange pathway 40 of the refrigeration zone 28 where it is furthercooled and expanded via expander 48, preferably using a Joule-Thomsoneffect, thus providing cooling for the refrigeration zone 28 as mixedrefrigerant coolant. The mixed refrigerant coolant is passed through thesecond heat exchange pathway 42 where it is heated in countercurrentheat exchange with the compressed mixed refrigerant and the pre-treatedfeed gas passing through the first and third heat exchange pathways 40,44, respectively. The mixed refrigerant gas is then returned to therefrigerant suction drum 10 before entering the compressor 12, thuscompleting a closed loop single mixed refrigerant process.

Mixed refrigerant make-up is provided from the fluid material orboil-off gas (methane and/or C2-C5 hydrocarbons), nitrogen generator(nitrogen) with any one or more of the refrigerant components beingsourced externally.

The mixed refrigerant contains compounds selected from a groupconsisting of nitrogen and hydrocarbons containing from 1 to about 5carbon atoms. When the fluid material to be cooled is natural gas orcoal seam gas, a suitable composition for the mixed refrigerant is asfollows in the following mole fraction percent ranges: nitrogen: about 5to about 15; methane: about 25 to about 35; C2: about 33 to about 42;C3: 0 to about 10; C4: 0 to about 20 about; and C5: 0 to about 20. In apreferred embodiment, the mixed refrigerant comprises nitrogen, methane,ethane or ethylene, and isobutane and/or n-butane.

FIG. 2 shows a composite cooling and heating curve for the single mixedrefrigerant and natural gas. The close proximity of the curves to withinabout 2° indicates the efficiencies of the process and system of thepresent invention.

Additional refrigeration may be provided to the refrigeration zone 28 bythe auxiliary refrigeration system 20. The auxiliary refrigerationsystem 20 comprises one or more ammonia refrigeration packages cooled byair coolers. An auxiliary refrigerant, such as cool ammonia, passesthrough the fourth heat exchange pathway 44 located in a cold zone ofthe refrigeration zone 28. By this means, up to about 70% coolingcapacity available from the auxiliary refrigeration system 20 may bedirected to the refrigeration zone 28. The auxiliary cooling has theeffect of producing an additional 20% LNG and also improves plantefficiency, for example fuel consumption in gas turbine 100 by aseparate 20%

The auxiliary refrigeration system 20 utilises waste heat generated fromhot exhaust gases from the gas turbine 100 to generate the refrigerantfor the auxiliary refrigeration system 20. It will be appreciated,however, that additional waste heat generated by other components in theliquefaction plant may also be utilised to regenerate the refrigerantfor the auxiliary refrigeration system 20, such as may be available aswaste heat from other compressors, prime movers used in powergeneration, hot flare gases, waste gases or liquids, solar power and thelike.

The auxiliary refrigeration system 20 is also used to cool the air inletfor gas turbine 100. Importantly, cooling the gas turbine inlet air adds15-25% to the plant production capacity as compressor output is roughlyproportional to LNG output.

The liquefied gas is recovered from the third heat exchange pathway 44of the refrigeration zone 28 through a line 72 at a temperature fromabout -150° C. to about -170° C. The liquefied gas is then expandedthrough expander 74 which consequently reduces the temperature of theliquefied gas to about -160° C. Suitable examples of expanders which maybe used in the present invention include, but are not limited to,expansion valves, JT valves, venturi devices, and a rotating mechanicalexpander.

The liquefied gas is then directed to storage tank 76 via line 78.

Boil-off gases (BOG) generated in the storage tank 76 can be charged toa compressor 78, preferably a low pressure compressor, via line 80. Thecompressed BOG is supplied to the refrigeration zone 28 through line 82and is passed through a portion of the refrigeration zone 28 where saidcompressed BOG is cooled to a temperature from about -150° C. to about-170° C.

At these temperatures, a portion of the BOG is condensed to a liquidphase. In particular, the liquid phase of the cooled BOG largelycomprises methane. Although the vapour phase of cooled BOG alsocomprises methane, relative to the liquid phase there is an increase inthe concentration of nitrogen therein, typically from about 20% to about60%. The resultant composition of said vapour phase is suitable for useas a fuel gas.

The resultant two-phase mixture is passed to a separator 84 via line 86,whereupon the separated liquid phase is redirected back to the storagetank 76 via line 88.

The cooled gas phase separated in the separator 84 is passed to acompressor, preferably a high pressure compressor, and is used in theplant as a fuel gas and/or regeneration gas via line.

Alternatively, the cooled gas phase separated in the separator 84 issuitable for use as a cooling medium to circulate through a cryogenicflowline system for transfer of cryogenic fluids, such as for exampleLNG or liquid methane from coal seam gas, from a storage tank 76 to areceiving/loading facility, in order to maintain the flowline system ator marginally above cryogenic temperatures.

Referring to FIG. 1, there is shown a main transfer line 92 and a vapourreturn line 94, both fluidly connecting storage tank 76 to aloading/receiving facility (not shown). Storage tank 76 is provided witha pump 96 for pumping LNG from storage tank 76 through the main transferline 92.

As described previously, the cooled gas phase separated in the separator85 is suitable for use as a cooling medium to circulate through acryogenic flowline system for transfer of cryogenic liquids.Accordingly, the cooled gas phase separated in the separator 85 isdirected via line 98 to the main transfer line 92, whereupon the cooledgas phase is circulated through the main transfer line 92 and the vapourreturn line 94 to maintain the cryogenic flowline system at atemperature at or marginally above cryogenic temperatures.

Preferably, the vapour return line 94 is fluidly connected to an inletof the compressor 78 so that boil-off gases generated during transferoperations may be conveniently treated in accordance with the processfor treating boil-off gases as outlined above.

Before transfer operations commence, it is envisaged that additionalcooling and filling of the main transfer line could be achieved bypriming said line 92 by passing the liquid phase separated in separator84 or liquid fluid material discharged from heat exchanger 28 throughsaid line 92 via line 99. It is anticipated that any liquid phaseremaining in the line 99 after completion of transfer operations couldself-drain back into the storage tank 76 under inherent pressureself-generated in the line 99 from ambient heating.

The process and system described above has the following advantages overtraditional LNG plants:

-   -   (1) Integrated combined heat and power technology systems (CHP)        use waste heat from the gas turbines 100 plus some auxiliary        firing with recovered boil-off gas (which is low Btu waste gas)        to provide all heating requirements and electrical power via a        steam turbine generator for the LNG plant. The waste heat is        also used to drive standard packaged ammonia refrigeration        compressors of the auxiliary refrigeration system 20 which        provides additional refrigeration for:    -   gas turbine inlet air cooling, thereby improving plant capacity        by 15-25%;    -   general process cooling, thereby reducing the size of the        dehydration plant and balancing regeneration gas with the fuel        gas required to power the gas turbines 100;    -   additional cooling for the refrigeration zone, thereby improving        plant production capacity by up to 20% and energy efficiency by        up to another 20%;    -   (2) The mixed refrigerant system is designed to provide a close        match on the cooling curves thereby maximising refrigeration        efficiency. Integration of the auxiliary refrigeration system 20        with the refrigeration zone 28 improves the heat transfer at the        warm end of the heat exchanger by increasing the LMTD which        reduces the size of the heat exchanger. This also provides a        cool mixed refrigerant suction temperature to the compressor        which significantly improves the compressor capacity.    -   (3) The high efficiency, use of CHP to meet all plant heat and        electrical power requirements and the use of dry low emissions        combustors in the gas turbines 100 results in very low overall        emissions.    -   (4) Efficient BOG recovery. The system is configured to recover        flash gas and BOG generated from the storage tank 76 and from        the receiving/loading facility (eg. ships) during loading. The        BOG gas is compressed in compressor 78 where it is re-liquefied        in the refrigeration zone 28 to recover methane as liquid. The        liquid methane is returned to the storage tank 26 and the flash        gas which is concentrated in nitrogen is used to auxiliary fire        the exhaust of the gas turbine 100. This is a cost effective and        energy efficient way of dealing with BOG and rejecting nitrogen        from the system, and at the same time minimise or eliminate        flaring during loading.    -   (5) Efficient transfer flowline system. The system is configured        to provide a reduction in heat loss from the transfer lines and        a concomitant reduction in BOG generated therein, a portion of        which would be flared under prior art conditions. In the present        invention, any BOG which is generated in the transfer flowlines        may be recirculated to the compressor 78 and refrigeration zone        28 for liquefaction, and use as a cooling medium. Additionally,        the process and system obviates the need for an additional        transfer lines and associated pumps for circulation, thus        reducing the capital expenditure of said system.    -   (6) Lower plant capital and operating/maintenance costs. Fewer        equipment items and modular packages results in reduced civil,        mechanical, piping, electrical and instrumentation works and a        fast construction schedule; all of which contribute to reduced        costs. This results in simple operations requiring less        operating and maintenance staff.

It is to be understood that, although prior art use and publications maybe referred to herein, such reference does not constitute an admissionthat any of these form a part of the common general knowledge in theart, in Australia or any other country.

For the purposes of this specification it will be clearly understoodthat the word “comprising” means “including but not limited to”, andthat the word “comprises” has a corresponding meaning.

Numerous variations and modifications will suggest themselves to personsskilled in the relevant art, in addition to those already described,without departing from the basic inventive concepts. All such variationsand modifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription.

1. A process for liquefying a hydrocarbon gas comprising the steps of:a) pre-treating a hydrocarbon feed gas to remove sour species and watertherefrom; b) providing a refrigeration zone, wherein refrigeration inthe refrigeration zone is provided by circulating a mixed refrigerantfrom a mixed refrigerant system and an auxiliary refrigerant from anauxiliary refrigeration system through the refrigeration zone; c)coupling the mixed refrigerant system and the auxiliary refrigerationsystem in a manner whereby the auxiliary refrigeration system is driven,at least in part, by waste heat generated by the mixed refrigerant; andd) passing the pre-treated feed gas through the refrigeration zone wherethe pre-treated feed gas is cooled and expanding the cooled feed gas toproduce a hydrocarbon liquid.
 2. The process according to claim 1,wherein the step of circulating a mixed refrigerant through therefrigeration zone comprises: a) compressing the mixed refrigerant in acompressor; b) passing the compressed mixed refrigerant through a firstheat exchange pathway extending through the refrigeration zone where thecompressed mixed refrigerant is cooled and expanded to produce a mixedrefrigerant coolant; c) passing the mixed refrigerant coolant through asecond heat exchange pathway extending through the refrigeration zone toproduce a mixed refrigerant; and d) recirculating the mixed refrigerantto the compressor.
 3. The process according to claim 2, wherein the stepof passing the pre-treated feed gas through the refrigeration zonecomprises passing the pre-treated feed gas through a third heat exchangepathway in the refrigeration zone.
 4. The process according to claim 2,wherein the step of circulating the auxiliary refrigerant through therefrigeration zone comprises passing the auxiliary refrigerant through afourth heat exchange pathway extending through a portion of therefrigeration zone.
 5. The process according to claim 4, wherein thesecond and fourth heat exchange pathways extend in countercurrent heatexchange relation to the first and third heat exchange pathways.
 6. Theprocess according to claim 2, wherein the waste heat is produced fromthe compressing step.
 7. The process according to claim 2, wherein theprocess further comprises cooling inlet air of a gas turbine directlycoupled to the compressor with the auxiliary refrigerant.
 8. The processaccording to claim 7, wherein the inlet air is cooled to a temperaturein a range of about 5° C.-10° C.
 9. The process according to claim 2,wherein the step of compressing the mixed refrigerant increases thepressure thereof from about 30 to 50 bar.
 10. The process according toclaim 2, wherein the process comprises cooling the compressed mixedrefrigerant prior to passing the compressed mixed refrigerant to thefirst heat exchange pathway.
 11. The process according to claim 10,wherein the compressed mixed refrigerant is cooled to a temperature lessthan 50° C.
 12. The process according to claim 10, wherein thecompressed mixed refrigerant is cooled to about 10° C.
 13. The processaccording to claim 10, wherein the step of cooling the compressed mixedrefrigerant comprises passing the compressed mixed refrigerant from thecompressor to a heat exchanger.
 14. The process according to claim 13,wherein the heat exchanger is an air- or water-cooler.
 15. The processaccording to claim 13, wherein the cooling step comprises passing thecompressed mixed refrigerant from the compressor to the heat exchangerand further passing the compressed mixed refrigerant cooled in the heatexchanger to a chiller.
 16. The process according to claim 15, whereinthe chiller is driven at least in part by waste heat.
 17. The processaccording to claim 16, wherein the waste heat is produced from thecompressing step.
 18. The process according to claim 2, wherein thetemperature of the mixed refrigerant coolant is at or below thetemperature at which the pre-treated feed gas condenses.
 19. The processaccording to claim 18, wherein the temperature of the mixed refrigerantcoolant is less than -150° C.
 20. The process according to claim 1,wherein the mixed refrigerant contains compounds selected from a groupconsisting of nitrogen and hydrocarbons containing from 1 to 5 carbonatoms.
 21. The process according to claim 20, wherein the mixedrefrigerant comprises nitrogen, methane, ethane or ethylene, isobutaneand/or n-butane.
 22. The process according to claim 20, wherein thecomposition of the mixed refrigerant is in the following mole fractionpercent ranges: nitrogen: about 5 to about 15; methane: about 25 toabout 35; C2: about 33 to about 42; C3: 0 to about 10; C4: 0 to about 20about; and C5: 0 to about
 20. 23. The process according to claim 1,wherein the hydrocarbon gas is natural gas or coal seam methane.
 24. Theprocess according to claim 23, wherein the hydrocarbon gas is recoveredfrom the refrigeration zone at a temperature at or below theliquefaction temperature of methane.
 25. A hydrocarbon gas liquefactionsystem comprising: a) a mixed refrigerant; b) a compressor forcompressing the mixed refrigerant; c) a refrigeration heat exchanger forcooling a pre-treated feed gas to produce a hydrocarbon liquid, therefrigeration heat exchanger having a first heat exchange pathway influid communication with the compressor, a second heat exchange pathway,and a third heat exchange pathway, the first, second and third heatexchange pathways extending through the refrigeration zone, and a fourthheat exchange pathway extending through a portion of the refrigerationzone, the second and fourth heat exchange pathways being positioned incounter current heat exchange in relation to the first and third heatexchange pathways; an expander in fluid communication with an outletfrom the first heat exchange pathway and an inlet to the second heatexchange pathway; d) a recirculation mixed refrigerant line in fluidcommunication with an outlet from the second heat exchange pathway andan inlet to the compressor; e) an auxiliary refrigeration system havingan auxiliary refrigerant in fluid communication with the fourth heatexchange pathway; f) a source of pre-treated feed gas in fluidcommunications with an inlet of the third heat exchange pathway; and g)a hydrocarbon liquid line in fluid communication with an outlet of thethird heat exchange pathway.
 26. The system according to claim 25,wherein the compressor is a single stage compressor driven by a gasturbine.
 27. The system according to claim 26, wherein the compressor isa single stage centrifugal.
 28. The system according to claim 26, thecompressor is a two stage compressor driven by respective gas turbineswith intercooler and interstage scrubber.
 29. The system according toclaim 26, wherein the gas turbine is coupled with a steam generator in aconfiguration whereby, in use, waste heat from the gas turbinefacilitates production of steam in the steam generator.
 30. The systemaccording to claim 29, wherein the steam generator is coupled to asingle steam turbine generator configured to produce electrical power.31. The system according to claim 30, wherein the amount of electricalpower generated by the single steam turbine generator is sufficient todrive the auxiliary refrigeration system.
 32. The system according toclaim 25, wherein the auxiliary refrigerant comprises low temperatureammonia and the auxiliary refrigeration system comprises one or moreammonia refrigeration packages.
 33. The system according to claim 32,wherein the one or more ammonia refrigeration packages are cooled by aircoolers.
 34. The system according to claim 26, wherein the auxiliaryrefrigeration system is in heat exchange communication with the gasturbine, the heat exchange communication being configured in a manner toeffect cooling of inlet air of the gas turbine by the auxiliaryrefrigeration system.
 35. The system according to claim 25, wherein thesystem comprises a cooler to cool the compressed mixed refrigerant priorto the compressed mixed refrigerant being received in the refrigerationheat exchanger.
 36. The system according to claim 35, wherein the cooleris an air-cooled heat exchanger, or a water-cooled heat exchanger. 37.The system according to claim 25, wherein the hydrocarbon liquid in thehydrocarbon liquid line is expanded through an expander to further coolthe hydrocarbon liquid.